1. Field of the Invention
The present invention relates generally to a liquid crystal material displays (LCDs) and more specifically to polymer dispersed liquid crystal (PDLC) displays comprising a composite layer of liquid crystal material and polymer, and to a method of fabricating LCDs useful in computer displays, video projectors, and the like. There are two types of PDLC displays, normal mode and reverse mode. The present invention, in particular, is directed to the reverse mode PDLC display wherein the application of an applied voltage results in an opaque or ON state for the display.
2. Description of the Related Art
Liquid crystal material displays can be classified into at least two types that employ polymer matrices encapsulating liquid crystal material globules: the nematic curvilinear aligned phase (NCAP) type and the polymer dispersed liquid crystal material (PDLC) type. An example of NCAP is a material made by Taliq Corporation, a subsidiary of Raychem Corporation, Menlo Park, Calif., and is available in large sheets because the liquid crystal material does not need to be sealed in between precisely spaced substrates. The PDLC type is further separable into normal mode and reverse mode displays. With normal PDLC displays, a liquid crystal material is distributed in droplets in a polymer matrix. Considerable research on normal PDLC material has been conducted by Kent State University, General Motors Corporation, Hughes Aircraft, Seiko Epson Corporation (Japan), Asahi Glass (Japan), Dainippon Ink and Chemical (Japan), and others. Doane, et al. have received U.S. Pat. No. 4,994,204, issued Feb. 19, 1991, relating to PDLC. Doane, et al. claim their liquid crystal material microdroplet structure and birefringent light transmissive synthetic resin matrix can be manipulated to obtain effects different from normal mode and reverse mode. In reverse mode PDLC displays, the devices are translucent milky white with the application of an applied voltage.
By now, most people are familiar with liquid crystal material displays (LCDs). They are ubiquitous in watches, calculators, laptop computers, and small TVs. Light emitting diodes (LED) are an early technology used in watches and calculators that ultimately gave way to LCDs, because the power needed to run LEDs is too high and LEDs are hard to read in bright sunlight. LCDs are used a lot because flat, thin display can be constructed with relatively large areas. The laptop computers generally take advantage of this feature. Projection TV, which once used high-output CRT tubes, now is even brighter and more compact, thanks to the use of transmissive mode LCDs set in front of halogen projection lamps.
U.S. Pat. No. 4,613,207, issued Sep. 23, 1986, to Fergason describes a liquid crystal material projector and method. A projection lamp has its light collected by a collimator lens and directed to a liquid crystal material display that passes or blocks the light according to a modulation signal applied to a transmissive mode LCD. Light that passes through the LCD is focused by an optics system for viewing on a screen. Fergason suggests using dye to color light to produce a colored output.
The most popular LCDs are the twisted nematic type, which use two light polarizing plates in order to achieve a contrast between field-ON and field-OFF conditions. Liquid crystal material in contact with a substrate that has been rubbed in a uniform direction will align with that direction. If the liquid crystal material, in its liquid phase form, is contained between the two substrates that have surfaces that have been rubbed at 90.degree. relative to each other, successive layers of liquid crystal material molecules will twist so that they align with the rubbing direction at each liquid crystal material/substrate interface. The relative twist between such layers is gradual enough that polarized light can twist correspondingly along with the layers and exit polarized 90.degree. from its original orientation. If polarizing plates are positioned properly at each substrate, light can pass directly through the assembly, rendering the whole assembly to appear transparent. However, if an electric field is placed across the liquid crystal material, its influence on the alignment of the liquid crystal material therebetween and its molecular layers will be stronger than that provided by the rubbing alignment at the substrates. In a field-ON condition, the twist structure will be straightened or rendered uniform, and light passing through the structure will not be twisted sufficiently to pass through the second polarizing plate. The area involved in the field, therefore, will appear dark or black. Unfortunately, these polarizing plates substantially reduce the amount of light that is able to pass through an LCD. Most light sources do not directly produce polarized light. All undesirably polarized light is filtered out. Only a small percentage will have the proper polarization and will pass through the liquid crystal material. To compensate, higher power backlight or projection lamps must be used, or the resulting reduced light levels must simply be tolerated.
As a solution to the above mentioned shortcomings, a type of PDLC display has appeared requiring no polarizing plates and employs the difference between the refractive indices of a liquid crystal material and a polymer having liquid crystal properties which are mixed together but phase separated, i.e., one component remains in a liquid phase and the other component remains in a solid phase. The optical properties of the liquid crystal material and the liquid crystal polymer can be made to match or not match so that the refractive indices of these separate components are nearly matched or not matched. When their indices are matched, the display is transparent. When their indices are not matched, the display becomes translucen, milky white.
FIG. 58 illustrates an example of a known PDLC display, also shown in U.S. Pat. No. 3,600,060. This LCD has a composite layer which is sandwiched between the two transparent substrates 01 and 08 and have transparent electrodes 02 and 07 on their inner surfaces. The composite layer has microscopic grains of a liquid crystal material 05 distributed throughout a solid, sponge-like polymer matrix 04. The molecules of polymer matrix 04 are randomly oriented. Liquid crystal material 05 has a positive dielectric anisotropy and its molecules will align in the direction of an applied electric field. Under field-OFF conditions, as shown in FIG. 58A, molecules of liquid crystal material 05 in the various pockets of polymer matrix 04 will be randomly aligned. The liquid crystal material has a refractive index of approximately 1.6, which is an average of the normal refractive index, which is about 1.5, and the extraordinary refractive index, which is about 1.7. The polymers in polymer matrix 04 are solidified, e.g., polymerized, without regard to the constituent individual molecules' alignment. Their resulting refractive index is approximately 1.5. In FIG. 58, the directions of the arrows in polymer matrix 04 indicate random orientations in three dimensions. Under field-OFF condition, there is a difference in the refractive index of about 0.1 at the countless interfaces between liquid crystal material 05 and polymer matrix 04. These interfaces are envelopes containing the microdroplets of liquid crystal material 05. Light incident to the composite layer will be scattered, making the display appear to be translucent, milky white or translucent. Actually, only incident light polarized parallel to the direction of alignment of the molecules of liquid crystal material will be scattered. When a drive signal is connected between the transparent electrodes 02 and 07, as shown in FIG. 58B, applying an electric field to liquid crystal polymer composite layer, individual molecules of liquid crystal material 05 re-align themselves in the direction of the electric field. A refractive index of about 1.5 results, which approximates the refractive index of polymer matrix 04. Incident light will, therefore, pass through the composite layer without being scattered, so that the display is transparent.
Describe in U.S. Pat. No. 4,944,576 to Lacker, et al, issued Jul. 31, 1990, is a PDLC fabrication method that enables partial pre-alignment of the liquid crystal material. Partial alignment is attained by the controlled application of an electric or magnetic field, or a mechanical flow, during the polymerization process.
Although the above description gives an example where the display becomes translucent, milky white under field-OFF conditions and transparent with an applied electric field under field-ON conditions, it is possible to construct a device with opposite optical characteristics. Other types and combinations of liquid crystal material and polymers are employed to achieve this opposite characteristic. If the refractive indices of liquid crystal material and polymer are adjusted to be equal or close to each other in the absence of an electric field, incident light passes through the composite layer without scattering so that the display appears transparent. When liquid crystal material is aligned because of an applied electric field, differences between the refractive indices of liquid crystal material and polymer is increased, thereby scattering light at the interface between liquid crystal material and polymer. As a result, the display appears translucent, milky white or translucent.
Another type of PDLC display which becomes transparent or translucent, milky white on the application of an applied field is based on a different principle. This other type of display relies on a phenomenon where the display clouds by applying an electric field to a composite layer comprising a mixture of polymer, such as, a methacrylate with a side chain having a biphenyl molecular structure and a metamorphosis of the liquid crystal material is caused by an exposure to strong ultraviolet light.
A liquid crystal polymer composite display with dichroic dye has been reported in 1990 by the Society for Information Display in the International Symposium Digest of Technology Papers, No. 12, May 1, 1990. Dichroic dye is added to liquid crystal material in a composite layer sandwiched between two transparent electrodes. Under field-OFF conditions, incident light will be scattered due to a difference in the refractive indexes between the liquid crystal material and the polymer. When the dichroic dye is randomly aligned, scattered light permits the dye color to be visually observed. Under field-ON conditions, a liquid crystal material with the dye aligns in the direction of the field, effectively zeroing out any difference in refractive indices at the interface between liquid crystal material and polymer. The display becomes transparent. It is customary to place a colored sheet of paper as a background for such a display.
Conventional PDLC display elements have microscopic grains of a liquid crystal material dispersed in a randomly aligned polymer which prevent the appearance of complete transparency. Since the sizes of these grains are not uniform, the display quality is not uniformly consistent across the display resulting in low reliability. Further, under field-OFF condition, the liquid crystal material becomes randomly aligned. Also, individual liquid crystal material molecules will not have a uniform response to an electric field, which prevents the display from having a sharp switch-over in ON/OFF states.
There is relatively a small difference between the refractive index of the polymer (1.5) and the average refractive index of the liquid crystal material (1.6). A sufficient amount of light scattering effect is not achieved for a completely translucent, milky white condition, even when the refractive index of liquid crystal material is different from the polymer. If the composite layer is made thicker to improve the amount of the light scattering effect, the required applied power to drive the display would have to be raised to the range of 60 to 80 volts.
Prior art displays have slow or poor threshold characteristics resulting in improper amounts of contrast. For example, if the maximum number of scan lines is three for simple matrix driving, it is necessary to employ an active element, such as, a TFT (thin film transistor) element or MIM (metal-insulator-metal) element when driving a display having a large viewing screen area.
Also, in a polymer LCD having a dichroic dye, the transparent mode and/or dye color is changed by the turning an electric field on or off. However, the display will appear dark. Further, increasing the amount of dye content will overall darken the display and also require a higher driving voltage.
To overcome these shortcomings, a PDLC display element has recently been developed eliminating the need for polarizing plates and employing differences between refractive indices of combined components comprising liquid crystal material and a polymer to provide contrast conditions for display of information. Polymer dispersed LCDs have a combination liquid crystal material and a polymer and are phase separated from one another. The operation of this type of display is such that when refractive indices of both these components match for, respectively, ON or OFF field conditions, the display will appear transparent. When the refractive indices of these components are no longer the same because of, respectively, field ON or OFF conditions, the display will appear translucent, milky white.
An object of this invention is the provision of a PDLC display having commonly aligned liquid crystal material and polymer components insuring excellent threshold characteristic, good contrast and a high brightness.
Another object of this invention is the provision of a reflection-type PDLC display which requires a relatively low drive voltage having good contrast and visibility, providing a large area display.
A further object of this invention is the provision of a PDLC display having a high specific resistance and excellent charge holding characteristic.
A still further object of this invention is a method of fabricating PDLC display elements.